The present invention relates to an improved apparatus for forming blanks from which optical waveguide fibers are drawn.
Certain glass making processes, particularly vapor deposition processes, have been commonly employed in the formation of optical waveguide blanks. In one such process, the source material vapor is directed into a heated tube wherein it reacts to form a material which is deposited in successive layers. The combination of deposited glass and tube is collapsed to form a draw blank which can be later heated and drawn into an optical waveguide fiber.
In order to obtain uniform deposition along the length of the substrate tube, a serial deposition process has been employed. That is, reactants are fed into the end of the tube, but deposition occurs only in a narrow section of the tube immediately downstream from the zone which is heated by a flame. The flame moves up and down the tube to move the reaction and thus the region of glass deposition serially along the tube.
One of the limitations of such a process is a comparatively low effective mass deposition rate. To increase the deposition rate it appears to be necessary to increase the inside diameter of the substrate tube to provide a greater collection surface area. However, since heat is supplied from the outside of the tube, a larger tube diameter results in a lower vapor temperature at the axis of the tube. Moreover, the flow profile across the tube is such that maximum flow occurs axially within the tube. As tube diameter increases, a smaller portion of the reactant vapor flows in that region of the tube adjacent the wall where reaction products are more readily collected on the inner surface of the tube.
Various attempts have been made to increase the deposition efficiency and the rate at which glass is deposited on the inner surface of the bait tube. In U.S. Pat. No. 4,117,802 a hollow cylindrical element is inserted into the downstream end of the bait tube, the closed end of the cylindrical element terminating in the hot zone. The reactants are caused to flow around the cylindrical element and are therefore channeled close to the heated bait tube. Thus, a greater portion of the reactants are said to react and form a part of the glassy deposit on the inner surface of the bait tube. A coolant gas flows through the cylindrical element for the stated purpose of reducing the amount of glass deposited on its surface. It has been found that the effect of the coolant gas would be to increase the deposition of glass soot on the cylindrical member, thus reducing the amount of glass soot available to be deposited on the inner surface of the bait tube and eventually adversely restricting the flow of gases through the annular channel between those two cylindrical members.
An apparatus for increasing the efficiency of glass deposition in a vapor deposition process for making optical waveguide preforms is taught in U.S. patent application Ser. No. 913,754 filed June 8, 1978, now abandoned, and entitled "Method of Making Large Diameter Optical Waveguide Preforms", the corresponding West German application having been published Dec. 13, 1979 as No. 2,922,795. A baffle tube extends into that end of the bait tube into which the reactants flow. The baffle tube, which traverses the bait tube along with the burner, ends just short of the hot zone so that no soot is deposited thereon. A gas flowing from the baffle tube creates a gaseous mandrel which confines the flow of reactant vapors to an annular channel adjacent the bait tube wall in the hot zone, thereby increasing deposition rate and efficiency.
Another apparatus which was developed for the purpose of increasing the efficiency of glass deposition in a vapor deposition process is taught in U.S. patent application Ser. No. 963,837 filed Nov. 27, 1978 and entitled "Apparatus and Method for Making Optical Filament Preform", now U.S. Pat. No. 4,233,045. A reactant feed tube extends into one end of the bait tube and terminates just short of the hot zone where reaction occurs. The end of the feed tube traverses the bait tube along with the burner which generates the hot zone. Reactants flow radially from slots in the end of the wall of the feed tube and combine with a flushing gas to form a mixture which flows in a spiral path through the hot zone.
U.S. patent application Ser. No. 38,775 filed May 14, 1979 and entitled "Optical Waveguide Manufacturing Process and Article", now U.S. Pat. No. 4,235,616, teaches another apparatus for increasing deposition efficiency. A hollow cylindrical substrate is provided within which is disposed a burner having a substantially radial flame. Reactants are flowed into the hollow cylindrical substrate in the annular space between the inner surface of the substrate and the exterior of the burner. A hot zone is established within the interior of the substrate in the vicinity of the radial flame so that the reactants are reacted to produce a suspension of particulate material. Also, a shield may be provided surrounding the burner within the hollow substrate, and a stream of gas flowed within the shield around the burner to confine the flow of particulate material to an annular channel adjacent to the inner surface of the substrate increasing deposition efficiency of the particulate material on the inside surface of the substrate.
It is therefore an object of the present invention to improve the deposition efficiency of a process whereby a reactant vapor flows into and reacts within a heated tube to form a layer therein.
Another object is to provide an efficient vapor deposition process which is not subject to flow-impeding glass buildups that are inherent in certain prior art processes.